1. Field of the Invention
This invention relates, in general, to novel nickel-base single crystal alloys and, in particular, to such alloys having high strength at elevated temperatures. More specifically, the present invention relates to novel nickel-base single crystal alloys which retain their high temperature mechanical properties after prolonged or repeated exposure to elevated temperatures, the single crystal alloys being capable of being cast into desired shapes, such as turbine blades, vanes and other parts used in high temperature gas turbine engines. Even more specifically, the present invention relates to novel nickel-base single crystal alloys which can be coated with conventional coatings with an accompanying heat treatment to impart high temperature oxidation/sulfidation resistance thereto without the formation of deleterious phases at the alloy/coating substrate interface.
2. Description of the Prior Art
Schweizer et al., U.S. Pat. No. 4,222,794, discloses a nickel-base single crystal superalloy for use at elevated temperatures having a restricted composition consisting of 4.5-6.0% chromium, 5.0-5.8% aluminum, 0.8-1.5% titanium, 1.7-2.3% molybdenum, 4.0-6.0% tungsten, 5.5-8.0% tantalum, 1.0-5.0% rhenium, 0.2-0.6% vanadium, 0-7.0% cobalt and the balance nickel. This patent also discloses a method of heat treating the alloys described therein at a specific temperature range. Although the Schweizer et al patent discloses a single crystal alloy, said alloy differs chemically from the alloy of the present invention. For example, the alloy of the present invention is significantly higher in chromium content, titanium content and titanium to aluminum ratio and does not contain rhenium and vanadium.
Gell et al, U.S. Pat. No. 4,116,723 discloses single crystal nickel base superalloys free from intentional additions of cobalt, boron, and zirconium. Gell et al discusses the avoidance of the development in the single crystal alloys of deleterious phases after long term exposure at elevated temperatures (i.e. alloy instability), the phases being of two general types, sigma and mu. Sigma is undesirable because of its brittle nature while mu is undesirable because the phase ties up large amounts of the refractory solid solution strengtheners thus weakening the remaining alloy phases. The sigma and mu phases are termed TCP phases for topologically closed packed phases and one of their common properties is that they all contain cobalt. Gell et al eliminates cobalt in the claimed single crystal nickel base alloys to inhibit the formation of TCP phases therein. Unexpectedly, the presence of cobalt in the single nickel base alloys of the present invention does not induce the formation of TCP phases. Also, the ratio of titanium to aluminum disclosed by Gell et al is lower than that in the alloy of the present invention.
Shaw, U.S. Pat. No. 4,207,098, discloses a relatively low-strength nickel-base polycrystalline alloy consisting essentially of 14-22% chromium, 5-25% cobalt, 1-5% tungsten, 0.5-3% tantalum, 2-5% titanium, 1-4.5% aluminum (with the sum of titanium plus aluminum being 4.5-9%), 0-2% niobium, 0.31-1.2% boron, 0-3.55 molybdenum, 0-0.5% zirconium, 0-0.2% in total yttrium or lanthanum or both, 0-0.1% carbon, and the balance nickel. The Shaw poly-crystalline alloy, which must contain boron, is chemically different from the single crystal alloy of the present invention.
Ghosh, U.S. Pat. No. 4,126,495, discloses a low strength nickel-base polycrystalline alloy consisting essentially of 6.75-10.0% aluminum, 8.0-12.0% chromium, 0.8-2.5% titanium, 2.0-6.0% cobalt, 2.5-4.0% molybdenum, 0.95-4.85% tantalum, 0-1.25% tungsten, 0-0.6% columbium, 0-1.0% carbon, 0-1.0% boron, 0-0.8% zirconium, 0-1.0% rare earths, 0-1.0% beryllium and the balance nickel. The Ghosh polycrystalline alloy contains lower amounts of tungsten and higher amounts of molybdenum than the single crystal alloy of the present invention.
Thielemann, U.S. Pat. No. 2,948,606, discloses a low-strength nickel-chromium-cobalt base polycrystalline alloy composed of about 15.0-25.0% chromium, 5.0-30.0% cobalt, 0.5-4.0% titanium, 2.0-5.0% aluminum, 1.0-5.0% columbium or tantalum or mixtures thereof, 5.0-11.0% tungsten and the balance essentially nickel. The Thielmann polycrystalline alloy which contains significantly higher amounts of chromium, a lower combined titanium-aluminum content and no molybdenum is chemically different from the single crystal alloy of the present invention.
Dalai et al, U.S. Pat. No. 3,807,993, discloses a polycrystalline material with a significantly higher cobalt content than the single crystal alloy of the present invention and, further, containing grain boundary strengtheners such as carbon, boron, zirconium and hafnium. The absence of these grain boundary strengtheners significantly increases the melting temperature and can allow higher heat treatment temperatures with attendant increases in strength. Moreover, higher melting temperatures allow higher engine use temperatures.
Two Restall et al patents, U.S. Pat. Nos. 3,902,900 and 3,922,168 disclose an intermetallic compound material containing a first group including nickel and at least one of the elements chromium, cobalt, molybdenum and tungsten within the range of 72-83 atomic percent and a second group containing aluminium (12-26 atomic percent) in combination with at least one of the elements titanium, niobium, and tantalum with the range of 17-28 atomic percent.
U.S. Pat. Nos. 4,249,943; 4,043,841; 3,785,809; 3,615,376; and 3,486,887 disclose alloys containing nickel, cobalt, chromium and aluminum together with one or more of the following elements: manganese, silicon, carbon, niobium, boron, zirconium among others.
U.S. Pat. Nos. 2,971,838; 3,276,866; 3,926,586; 3,973,952; and 4,268,308 disclose a variety of compositions containing nickel, chromium and aluminum with one or more of the following elements: zirconium, carbon, columbium, boron and silicon, among others.
Still other patents in the nickel base super-alloy area include U.S. Pat. Nos. 2,621,122; 2,781,264; 2,912,322; 2,994,605; 3,046,108; 3,166,412; 3,188,204; 3,287,110; 3,304,176; and 3,322,534.
Nickel-base superalloys which have been used over the years to fabricate gas turbine engine components typically contain, aside from certain levels of chromium, cobalt, aluminum, titanium and refractory metals (e.g., tungsten, molybdenum, tantalum and columbium) other elements such as carbon, boron and zirconium which act as grain boundary strengtheners.
Gas turbine blades are most commonly formed by casting and the process most often utilized produces parts having equiaxed non-oriented grains. Since the high temperature properties of metals are generally dependant upon grain boundary properties, efforts have been made to strengthen such boundaries, by addition of carbon, boron and/or zirconium, as discussed above, or to reduce or eliminate the grain boundaries transverse to the major stress axis of the part. One method of eliminating such transverse boundaries is directional solidification, described in U.S. Pat. No. 3,260,505. The effect of directional solidification is to produce an oriented microstructure of columnar grains whose major axis is parallel to the stress axis of the part and which has minimal or no grain boundaries perpendicular to the stress axis of the part. A further extension of this concept is the utilization of single crystal parts in gas turbine blades, as described in U.S. Pat. No. 3,494,709. The obvious advantage of the single crystal blade is the complete absence of grain boundaries as potential weaknesses. Thus, the mechanical properties of the single crystal are completely dependent upon the inherent mechanical properties of the material. While single crystal nickel-base alloys are generally known, there exists a need for such alloys having a combination of properties including improved mechanical strength, especially over prolonged and/or repeated exposure to elevated temperatures and the ability to be cast to desired shapes, such as turbine blades and parts.
While U.S. Pat. No. 4,116,723 relates to heat treatment of single crystal alloys, precipitation-hardened alloys having the high temperature mechanical properties of the instant invention (e.g., retention of high temperature properties after prolonged or repeated exposure to elevated temperatures) are not obtained.
It is therefore, an object of the present invention to provide a novel alloy composition which is devoid of the above-noted disadvantages.
It is another object of this invention to provide a single crystal nickel base alloy composition which retains its high strength and exhibits long term phase stability after prolonged and/or repeated exposure to elevated temperatures.
It is a further object of this invention to provide a novel heat-treated, coated alloy composition with enhanced mechanical properties.
It is still another object of this invention to provide single crystal alloy compositions which are compatible with conventional high temperature coatings such as diffused aluminides and do not exhibit deleterious TCP phases at the coating/single crystal alloy interface.
It is yet a further object of this invention to provide a novel high strength, single crystal nickel base alloy which may be cast to desired shapes, such as turbine blades and other parts used in high temperature gas turbine engines.
It is yet another objective of this invention to provide a novel nickel base single crystal alloy composition having exceptional coated oxidation and sulfidation resistance and high strength at elevated temperatures.
It is yet a further object of this invention to provide a novel high strength single crystal alloy which may be cast to desired shapes (for example turbine blades, vanes or other parts) and can be used as coated or uncoated parts in high temperature gas turbines.